|
Sexual differentiation in humans is the process of development of sex differences in humans. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination.〔Hughes, Ieuan A. . (June 12, 2011). Minireview: Sex Differentiation. Available: http://endo.endojournals.org/content/142/8/3281.full. Last accessed May 21, 2011.〕 Sexual differentiation includes development of different genitalia and the internal genital tracts, breasts, body hair, and plays a role in gender identification.〔http://www.gfmer.ch/Books/Reproductive_health/Human_sexual_differentiation.html〕 The development of sexual differences begins with the XY sex-determination system that is present in humans, and complex mechanisms are responsible for the development of the phenotypic differences between male and female humans from an undifferentiated zygote. Females have two X chromosomes, and males have a Y chromosome and an X chromosome. At an early stage in embryonic development, both sexes possess equivalent internal structures. These are the mesonephric ducts and paramesonephric ducts. The presence of the SRY gene on the Y chromosome causes the development of the testes in males, and the subsequent release of hormones which cause the paramesonephric ducts to regress. In females, the mesonephric ducts regress. Abnormal sexual development, and ambiguous genitalia, can be a result of genetic and hormonal factors. ==Sex determination== (詳細はbaby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be genetically female (XX) or male (XY). To be genetically female, one needs to be (XX), whereas to be a genetic male, (XY) is needed. It is the Y chromosome that is essential for the development of the male reproductive organs, and with no Y chromosome, an embryo will develop into a female. This is because of the presence of the sex determining region of the Y chromosome, also known as the SRY gene.〔Rey,Rodolfo, MD, PhD, Josso, Nathalie MD, PhD. (). Chapter 7. Sexual Differentiation. Available: http://www.endotext.org/pediatrics/pediatrics7/pediatrics7.html. Last accessed may 21, 2011.〕 A fetus doesn't develop its external sexual organs until -the fourth month of pregnancy —seven weeks after conception. The fetus appears to be sexually indifferent, looking neither like a male or a female. Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation. The precursor of the internal female sex organs is called the Müllerian system. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sexual differentiation in humans」の詳細全文を読む スポンサード リンク
|